1. Technical Field
The present invention relates to a light emitting display, a driving device thereof, and a driving method thereof. More specifically, the present invention relates to a light emitting display using an organic light emitting diode (OLED), and a driving method thereof.
2. Related Art
In general, an OLED display electrically excites a phosphorous organic compound to emit light, and it voltage-drives or current-drives a plurality of organic emitting cells to display images. The organic emitting cell includes an anode, an organic thin film, and a cathode layer.
A method for driving the organic emission cells can be classified as a passive matrix method or as an active matrix method using thin film transistors (TFTs). The passive matrix method provides anodes and cathodes that cross (or cross over) each other, in which method a line to drive the organic emission cells is selected. The active matrix method provides TFTs that access respective pixel electrodes, and a pixel is driven according to a voltage maintained by a capacitance of a capacitor accessed by a gate of a TFT. Depending on the formats of the signals applied to capacitors for establishing the voltage, the active matrix method can be categorized as a voltage programming method or a current programming method.
The pixel circuit of the voltage programming method has a difficulty in obtaining high gray scales because of deviations of the threshold voltage VTH and carrier mobility, the deviations being caused by non-uniformity in the manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scales in the case of driving thin film transistors by a voltage of 3V (volts), it is required to apply the voltage to the gate of the thin film transistor with an interval less than a voltage of 12 mV (=3V/256). If the deviation of the threshold voltage of the thin film transistor caused by the non-uniformity in the manufacturing process is 100 mV, it is difficult to represent high gray scales.
However, the pixel circuit of the current programming method achieves uniform display characteristics when the driving transistor in each pixel has non-uniform voltage-current characteristics, provided that a current source for supplying the current to the pixel circuit is uniform throughout the whole panel.
However, the pixel circuit of the current programming method involves a long data programming time because of a parasitic capacitance component provided on the data line. In particular, the time for programming the data on the current pixel line is influenced by the voltage state of the data line according to the data of a previous pixel line, and in particular, the data programming time is further lengthened when the data line is charged with a voltage which is very different from the target voltage (the voltage corresponding to the current data). This phenomenon becomes an even greater factor as the gray level becomes lower (near black).